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Rim joists - where spray foam interior and rigid board exterior meet.

I'm in a climate zone 5 area and have a 3' crawlspace with close to 6" of closed cell spray on the interior of the rim joists. I'm about to re-side possibly with Hardipanels, and insulate from the outside- planning on 2 layers of 1-1/2" of reclaimed polyso, held on with 3/4" furring strips over Tyvec for a rainscreen, well secured , enough to hold the Hardiboard. My intent is to run the siding roughly 1' below the top of the cement block. My question is- where do I end the polyso? I believe layering exterior polyso over top of what is already spray foamed from the interior could create moisture problems in the rim joist area. I'm also curious about what to do with the 3 & 3/4" (poly & furring strips) sticking out past the concrete block foundation. Is there a detail drawing that covers this?

Asked by stu m
Posted Wed, 07/09/2014 - 14:35

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Stu,
You wrote, "I believe layering exterior polyso over top of what is already spray foamed from the interior could create moisture problems in the rim joist area."

You would be creating a so-called "foam sandwich," with OSB or plywood sandwiched between two impermeable layers of foam. That's not ideal. If you had to do it again, it would have been better to use open-cell spray foam on the interior rather than closed-cell spray foam.

However, your sandwich will probably be OK, as long as the OSB or plywood sheathing and the rim joist are dry on the day that you cover them up with rigid foam. Choose a dry, sunny day for the work; you don't want to trap any more moisture in the sandwich than necessary.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 07/10/2014 - 08:28

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